Recent studies suggest a high volume of sedentary behavior may be a risk factor for adverse health outcomes.1 However, few data exist on how this behavior is patterned (eg, does most sedentary behavior occur in a few long bouts or in many short bouts?) and whether sedentary patterns are relevant for health. We examined details of sedentary behavior among older women. Because physical activity is influenced by age, body mass index (BMI; calculated as weight in kilograms divided by height in meters squared), and smoking status, we further examined sedentary behavior in relation to these characteristics.

METHODS

The Women’s Health Study is a completed randomized trial (1992-2004) of aspirin and vitamin E for preventing cardiovascular disease and cancer among 39 876 healthy women throughout the United States, with continuing observational follow-up (2004-present).2 An observational ancillary study, begun in 2011, is assessing physical activity using accelerometers. This cross-sectional study included all women who returned the accelerometer by March 2013. Women provided written consent to participate and the study was approved by the institutional review board of Brigham and Women’s Hospital. Women were mailed an accelerometer (ActiGraph GT3X+, ActiGraph Corp) and detailed instructions and were asked to wear it for 7 days during waking hours. They also completed a wear time diary, indicating which days the monitor was worn.

Accelerometer data were screened for wear time using standard methods.3 Briefly, nonwear time was defined as 90 consecutive minutes of zero counts, with an allowance of up to 2 minutes of nonzero counts, if there were 30-minute consecutive zero counts upstream and downstream. Wear days were identified using the diary. A bout of sedentary behavior was defined as consecutive minutes in which the accelerometer registered less than 100 counts per minute.4 A break in sedentary behavior was defined as at least 1 minute in which counts registered at least 100 following a sedentary bout. Using least-squares regression (SAS version 9.3, SAS Institute Inc), we tested for differences in mean values among subgroups using 2-sided tests with a significance level of P < .05.

RESULTS

Of 8373 women who returned the accelerometer by March 2013, we excluded 723 who did not return a diary and 403 who did not have at least 4 days of at least 10 hours of wear per day (standard convention)5; resulting in 7247 women (87%) with a mean (SD) age of 71.4 (5.8) years. Women wore the accelerometer for a mean (SD) of 14.8 (1.2) hours per day over a mean (SD) of 6.8 (0.57) days. The mean (SD) percentage of wear time spent in sedentary behavior was 65.5% (9.0%), equivalent to a mean (SD) of 9.7 (1.5) hours per day (Table 1). The mean (SD) number of sedentary bouts per day was 85.9 (16.1), with 9.0 (2.4) breaks per sedentary hour. Adjusting for wear time and smoking status, total sedentary time increased and the number of bouts and breaks per sedentary hour decreased as age and BMI increased (P < .001).

Table Graphic Jump LocationTable 1. Characteristics of Accelerometer-Assessed Sedentary Behavior Among All Women and in Subgroups in the Women's Health Study, 2011-2013

Most sedentary time occurred in bouts of shorter duration (Table 2). Among the total number of sedentary bouts, the mean (SD) percentage of bouts of at least 30 minutes was 4.8% (2.9%), representing 31.5% (12.4%) of total sedentary time.

DISCUSSION

This study provides a detailed analysis of sedentary behavior patterns among a large sample of older women, more than 6 times the size of previous similar studies.6 We found that older women spent about two-thirds of waking time in sedentary behavior, most of which occurred in bouts lasting less than 30 minutes. Previous studies have shown a similar proportion of time spent in sedentary behavior; however, these did not report on patterns.4,6

Even though accelerometers provide objective measures, they cannot convey postural information (ie, sitting vs standing). However, these older women are unlikely to be standing still for long periods. These data are limited to women from the Women’s Health Study, who are primarily white and of higher socioeconomic status; however, total sedentary time was similar to a national sample.4 If future studies confirm the health hazards of sedentary behavior and guidelines are warranted, these data may be useful to inform recommendations on how to improve such behavior.

Author Contributions: Mr Shiroma and Dr Lee had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Shiroma, Freedson, Lee.

Acquisition of data: Shiroma, Lee.

Analysis and interpretation of data: All authors.

Drafting of the manuscript: Shiroma, Trost, Lee.

Critical revision of the manuscript for important intellectual content: Freedson, Lee.

Statistical analysis: Shiroma, Lee.

Obtained funding: Lee.

Administrative, technical, or material support: All authors.

Study supervision: Lee.

Conflict of Interest Disclosures: The authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Drs Freedson and Trost are members of the Actigraph scientific advisory board. No other disclosures were reported.

Funding/Support: This research was supported by research grants CA154647, CA047988, CA121005, HL099557, HL043851, HL080467, HL099355, and HL007575 from the National Institutes of Health.

Role of the Sponsor: The National Institutes of Health had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Additional Contributions: We are grateful to the staff of the Women’s Health Study (Brigham and Women’s Hospital), particularly Ara Sarkissian, MM, Bonnie Church, BA, Colby Smith, and Jane Jones, MEd. None of the persons named were compensated.

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